Conventionally, rare-earth magnets having high energy densities such as neodymium sintered magnets have generally been used for permanent magnets of electric motors mounted in compressors of air conditioners in order to improve energy-saving performance of the air conditioners. Electric motors using neodymium sintered magnets have been developed for fans of air conditioners.
Such permanent magnets are expensive because the permanent magnets contain precious rare-earth elements. Therefore, there is a strong demand for cost reduction by reducing the usage amount and processing cost of permanent magnets.
In general, permanent magnets are made by cutting a block into a specified shape. Therefore, as the number of permanent magnets used in an electric motor increases, the processing cost increases.
As a method of reducing the number of permanent magnets used in an electric motor, there is a method of forming a rotor with so-called consequent poles. In a consequent-pole-type rotor, magnet magnetic poles produced by permanent magnets and salient poles not produced by permanent magnets but formed in a core material are alternately arrayed in the circumferential direction. Therefore, the number of the magnet magnetic poles and the number of the salient poles are each half the number of the poles. In addition, the magnet magnetic poles whose number is half the number of the poles have the same polarity, and the salient poles whose number is half the number of the poles have a polarity different from a polarity of the magnet magnetic poles. Thus, in the consequent-pole-type rotor, the number of the permanent magnets is half the usual number. However, in the consequent-pole-type rotor, the inductance differs between the magnet magnetic poles and the salient poles, and there is a problem that vibration and noise become large due to the inductance imbalance.
To this problem, Patent Literature 1 contrives a shape of flux barriers at both ends of each permanent magnet in the consequent-pole-type rotor, thereby to improve inductance asymmetry to reduce vibration and noise.